The present invention relates to apparatus for detecting acceleration, physical displacement, vibration, and the like. It finds particular application in conjunction with apparatus for sensing knocking in internal combustion engines and will be described with particular reference thereto. It is to be appreciated, however, that the invention is also applicable for sensing other physical conditions, particularly those which are marked by a predetermined, characteristic frequency, or frequency bandwidth, such as sensors for wheel shimmy, aircraft component vibration, bearing failures, and the like.
With the advent of high speed electronic engine control circuitry and the quest for greater internal combustion engine efficiency, internal combustion engines have come to be operated close to conditions with cause pre-ignition combustion or knocking. With changes in operating parameters, such as engine load, temperature, fuel, and the like, the internal combustion engine may begin to knock. Knocking, of course, sends vibration or shock waves through the engine which cause excessive engine wear, damage, and premature engine failure.
With adjustments in engine timing and other operating conditions, a knocking condition can be reduced or eliminated. Under some circumstances, a small amount of engine knock has been found to be desirable. To this end, others have suggested monitoring internal combustion engines for knock and adjusting timing and other operating parameters in order to eliminate knock or hold it in an acceptable range. One such system is shown in U.S. Pat. No. 4,474,472 issued Feb. 22, 1983 to Toshifumi Nishimura and another is described in U.S. Pat. No. 4,371,804 issued Feb. 1, 1983 to Yuchi Peng et al.
One of the problems with the above-referenced knock sensors is that they lack frequency selectivity. The Peng, et al. patent intentionally designs the sensor to be responsive to a broad range of frenquencies in order to accomodate a wide range of engine sizes and models. This broad frequency sensitivity increases the difficulty in distinquishing between engine knock and vibrations from other engine components, vehicle parts, road vibration, and the like. Responding to stray vibrations, as if they were engine knock, not only degrades engine performance, but can cause stalling, loss of power, and other potentially dangerous results.
The present invention provides an engine knock sensor which is responsive to a narrow frequency band to reduce sensitivity to stray engine and vehicle vibrations.